# Forces: Inertia & Newton's Third Law

## Forces: Inertia

### Definition of Inertia:

• Inertia is the property of an object that opposes changes in its state of motion.
• It is the tendency of an object to resist changes in its velocity, meaning an object will stay at rest or move in a straight line at a constant speed unless acted upon by an external force.
• Inertia is directly proportional to the mass of an object. A more massive object has greater inertia and requires more force to change its state of motion.

### Inertia and Newton’s First Law:

• Newton’s First Law of Motion, also known as the Law of Inertia, states that an object at rest stays at rest and an object in motion stays in motion with the same speed and direction unless acted upon by an external force.
• This law explains why seatbelts are important. If a car was to suddenly stop, the seatbelt provides the external force that stops the person from continuing to move forward due to their inertia.

## Forces: Newton’s Third Law

### Definition of Newton’s Third Law:

• Newton’s Third Law of Motion states that for every action (force) in nature, there is an equal and opposite reaction.
• In other words, any force exerted onto a body will create a force of equal magnitude but in the opposite direction on the object that exerted the first force.
• This is the reason why we move forward when we push off the ground to jump. The force we exert on the ground results in an equal force that the ground exerts on us, causing us to move upwards.

### Interacting Bodies:

• If two bodies are interacting, the forces each exerts on the other are always equal in magnitude and opposite in direction. These forces are known as interaction pairs.
• It’s important to note that interaction pairs do not cancel out as they are acting on different objects.
• Interaction pairs play an important role in the principle of action-reaction pairs which is another way of explaining Newton’s Third Law.